The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the acquisition of partial data sets from limited fields of view and the reconstructing of images from such data sets.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well-known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to sample so-called "k-space" and thereby produce a set of NMR data from which an image can be reconstructed. The phase encoding gradient G.sub.y steps from a negative value through zero to a corresponding positive value to sample k-space symmetrically around its origin.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts.
One method for reducing scan time is to reduce the total number of views acquired during the scan. The present invention relates to two such methods. One of these methods is to acquire only a portion of the views by sampling only a portion of k-space. Instead of sampling k-space symmetrically around the origin, only spatial frequencies on one side of the origin plus a small amount near the origin on the opposite side are sampled. For example, instead of stepping G.sub.y through 128 values ranging from -64 to +64, only the views ranging from -64 to +8 are acquired. As a result, fewer views are acquired which shortens scan time, but some k-space space data is missing from the acquired data set.
Such "partial" Fourier data acquisition usually uses Hermitian conjugate symmetry to replace the missing k-space data. Hermitian conjugate symmetry only works if the image is real. Numerous phase errors are present in MRI data that make the image complex. These phase errors result from phenomena such as B0 inhomogeneity, gradient eddy currents, group delays in the gradient amplifiers and receive electronics, and the spatial variation of surface coil receive B1 fields. To enable Hermitian conjugate replacement to work with a complex image, the replacement of the missing k-space data is accompanied by a phase correction which removes the phase errors from this data. One partial Fourier reconstruction algorithm, called "Homodyne reconstruction", uses two filters to accomplish the Hermitian conjugate replacement and the phase correction, respectively. A Homodyne high pass filter doubles the amplitude of the acquired k-space data which is conjugate to the missing k-space data prior to the Fourier transform. After the Fourier transform, the imaginary part of the image is discarded to complete the replacement step. The phase correction step is accomplished by a Homodyne low pass filter. This filter creates an image from a small portion of k-space data acquired symmetrically around the center of k-space. The phase of this image is subtracted from the phase of the Homodyne high pass filtered image prior to discarding the imaginary part of the image.
Another method for shortening the scan time without changing image resolution is to reduce the field of view of the acquired image. This enables fewer views to be acquired with a corresponding reduction in scan time. If the object being imaged extends beyond the field of view, however, aliasing, or wrap-around occurs in the phase encoding direction. To use this method to shorten scan time, therefore, a technique for suppressing, or canceling, the signals from spins outside the field of view must be used.
One technique for canceling signals outside the field of view is referred to as the "sense" technique. The sense technique employs two or more receiver coils which acquire two, separate, NMR signals from the region of interest. These two acquired NMR signals can be processed in combination with the known receive field function of each coil to produce an NMR signal without wrap-around. However, for this technique to function properly, the two acquired NMR signals must be complex signals containing both real and imaginary components.
There are applications where it is desirable to use both the partial k-space acquisition technique in combination with the sense technique. Unfortunately, however, the NMR signals produced by the Homodyne reconstruction must be stripped of their imaginary components because of the spurious phase shifts produced by this reconstruction method. Without two complex NMR signals to work with, the sense technique cannot be used.